Water In Reverse Micelles Research Articles

Vertical Variation in Dissolved Alkaline Phosphatase Activity in a Shallow Eutrophic Lake Determined in Reverse Micelles

ABSTRACT We evaluated the feasibility of microencapsulating dissolved alkaline phosphatase of a water body into reverse micelle systems prepared by hexadecyltrimethylammonium bromide as a surfactant in cyclohexane and 1-butanol as co-surfactant. The dissolved alkaline phosphatase activity within the micelle was described, including its kinetic parameters and the effects of pH and temperature on catalytic activity in surface, overlying and interstitial water of Lake Donghu. We found the similarities on the behavior of dissolved alkaline phosphatase of surface and interstitial water in reverse micelles, which was distinctly different from its behavior in the overlying water. This difference likely reflected the different origins of the dissolved alkaline phosphatase in the vertical profile of the lake. This system provides a novel tool with which to study the diversity and ecological significance of extracellular enzymes in aquatic environments.

Slow dynamics of water in reverse micelles

Nous presentons les variations avec le champ magnetique des vitesses de relaxation spin-reseau (R 1 ) des protons et deuterons de l'eau confinee dans des micelles inverses formees par le melange d'un tensioactif bis (2-ethylhexyl) sodium sulfosuccinate (AOT), de l'isooctane et de l'eau. Apres un plateau a tres bas champs, R 1 diminue en loi de puissance lorsque la frequence de Larmor augmente. Nous avons interprete ce comportement en frequence a l'aide d'un modele de relaxation de spin impliquant une reorientation moleculaire couplee a une translation en confinement spherique. Une simulation numerique de dynamique brownienne de molecules d'eau confinees dans une sphere est proposee et comparee aux resultats experimentaux. Cette etude constitue une etape preliminaire vers une meilleure comprehension de la dynamique lente de systemes biologiques en confinement.

Phase behaviors of AOT/longer chain n-alkanes reverse micelles in the presence of compressed ethylene

It was found that, in a suitable pressure range, ethylene could increase the amount of solubilized water in reverse micelles of sodium bis-2-ethylhexylsulfosuccinate (AOT) in longer chain n-alkanes considerably, where the phase separation was induced by a micelle–micelle interaction mechanism. The microenvironments in the ethylene-stabilized reverse micelles were investigated by the UV–vis adsorption spectra using methyl orange (MO) as a probe. The maximum absorption of MO decreased with the increase of ethylene pressure at constant W 0 value. Conductivity measurements demonstrated that the percolation temperature of the reverse micellar system increased considerably after compressed ethylene was added. The results of this work confirm that some small-molecule gases have the function of cosurfactants to stabilize reverse micelles.

Distribution of quencher ions in ternary system of Igepal/water/carbon tetrachloride at different water pool concentration: A spectroscopic study

A steady-state fluorescence quenching study has been undertaken to estimate aggregation numbers, N, and the average occupancy number ( n ¯ ) of quencher per micelle and the water pool size, r w, of nonionic surfactant Igepal (polyoxyethylene nonyl phenol), carbon tetrachloride and water reverse micelles at a different [water]/[Igepal] ratio. For the purpose, Safranine T (3,7-diamino-2,8-dimethyl-5-phenyl phenazinium chloride) has been employed as fluorescent probe and iodide, bromide and azide ions as static quencher. The fluorescence quenching data has been exploited to determine the nature and properties of water in the core of reverse micelles (RM), size of the water pool, association of quencher molecule with the RM, the localization of the probe and the distribution of quencher molecule in the quenching sphere. The demarcation of reverse micelles and microemulsion has also been determined.

Temperature dependence of hydrogen bonding and freezing behavior of water in reverse micelles.

The mid-infrared spectra of H2O and D2O confined in Aerosol OT (AOT) reverse micelles at various water/surfactant molar ratios (wo) were measured. Previous descriptions of reverse micellar (RM) water have identified three different hydrogen bonding populations in the water pool. (Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430-5434.) Fitting of the O-H and O-D stretching vibrational modes to Gaussian components corresponding to these three H-bonding populations was used to determine the temperature dependence of the hydrogen bonding populations and to observe the freezing behavior of the encapsulated water pool. The H-bond network behavior of the RM water pool exhibits a strong dependence on wo and does not approximate that of bulk water until wo = 40. The freezing temperature of RM water was wo-independent. The infrared spectra of frozen RM samples has also led us to suggest a mechanism for the low-temperature phase transition behavior of AOT reverse micelles, a subject of interest for cryoenzymology and low-temperature structural biology.

Dynamics of Nanoscopic Water: Vibrational Echo and Infrared Pump−Probe Studies of Reverse Micelles

The dynamics of water in nanoscopic pools 1.7-4.0 nm in diameter in AOT reverse micelles were studied with ultrafast infrared spectrally resolved stimulated vibrational echo and pump-probe spectroscopies. The experiments were conducted on the OD hydroxyl stretch of low-concentration HOD in the H2O, providing a direct examination of the hydrogen-bond network dynamics. Pump-probe experiments show that the vibrational lifetime of the OD stretch mode increases as the size of the reverse micelle decreases. These experiments are also sensitive to hydrogen-bond dissociation and reformation dynamics, which are observed to change with reverse micelle size. Spectrally resolved vibrational echo data were obtained at several frequencies. The vibrational echo data are compared to data taken on bulk water and on a 6 M NaCl solution, which is used to examine the role of ionic strength on the water dynamics in reverse micelles. Two types of vibrational echo measurements are presented: the vibrational echo decays and the vibrational echo peak shifts. As the water nanopool size decreases, the vibrational echo decays become slower. Even the largest nanopool (4 nm, approximately 1000 water molecules) has dynamics that are substantially slower than bulk water. It is demonstrated that the slow dynamics in the reverse micelle water nanopools are a result of confinement rather than ionic strength. The data are fit using time-dependent diagrammatic perturbation theory to obtain the frequency-frequency correlation function (FFCF) for each reverse micelle. The results are compared to the FFCF of water and show that the largest differences are in the slowest time scale dynamics. In bulk water, the slowest time scale dynamics are caused by hydrogen-bond network equilibration, i.e., the making and breaking of hydrogen bonds. For the smallest nanopools, the longest time scale component of the water dynamics is approximately 10 times longer than the dynamics in bulk water. The vibrational echo data for the smallest reverse micelle displays a dependence on the detection wavelength, which may indicate that multiple ensembles of water molecules are being observed.

Enhanced Stabilization of Reverse Micelles by Compressed CO2

The effect of compressed CO2 on the solubilization capacity of water in reverse micelles of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) in longer chain n-alkanes was studied at different temperatures and pressures. It was found that the amount of solubilized water is increased considerably by CO2 in a suitable pressure range. The suitable CO2 pressure range in which the solubilization capacity of water could be enhanced decreased with increasing W0 (water-to-AOT molar ratio). The microenvironments in the CO2-stabilized reverse micelles were investigated by UV/Vis adsorption spectroscopy with methyl orange (MO) as probe. The mechanism by which the reverse micelles are stabilized by CO2 is discussed in detail. The main reason is likely to be that CO2 has a much smaller molecular volume than the n-alkane solvents studied in this work. Therefore, it can penetrate the interfacial film of the reverse micelles and stabilize them by increasing the rigidity of the micellar interface and thus reducing the attractive interaction between the droplets. However, if the CO2 pressure is too high, the solvent strength of the solvents is reduced markedly, and this induces phase separation in the micellar solution.

Molecular Modeling and Simulations of AOT−Water Reverse Micelles in Isooctane: Structural and Dynamic Properties

This paper reports results from four nanosecond constant pressure and temperature simulations of sodium di-2-ethylhexylsulfoccinate (AOT) and water reverse micelles (RMs) in an apolar solvent, isooctane. The concentration of our simulated micelles was chosen to fall in a range which in nature corresponds to the L2 phase of the ternary system. To our knowledge, this is the first study to develop a full molecular model for AOT micelles in an apolar solvent. We address here the problems of the shape of the RM and of its hydrophilic inner core. For the AOT−water system, we obtain nonspherical aggregates of elliptical shape with ratios between major axis, a, and minor axis, c, between 1.24 and 1.41. The hydrophilic inner core is also ellipsoidal with larger a/c ratios. Although experiments indicate that the L2 of the AOT−water−oil system is likely to be polydisperse, we can only simulate monodisperse RMs. Nonetheless, our simulations are capable of reproducing well the dimensions of the water pool and their dependence on W0, as determined in some small-angle neutron and X-ray scattering experiments. Stimulated by recent experiments showing anomalous behavior of the confined water for AOT−water RMs, we have also investigated the static and dynamic properties of the RM's water inner core. From smaller micelles to larger, we find that the properties of confined water tend to near those of bulk water. In particular, we find that the solvation of the counterions is more effective in larger micelles and that diffusion of water is retarded with respect to bulk more in smaller RMs than in larger.

Fluorimetric determination of structural parameters of BuOH/SDS/H 2O reverse micelles

A steady-state fluorescence quenching study was undertaken to estimate aggregation numbers, N, and radii for the cell, r c, and for the water pool, r wp, of sodium dodecyl sulfate, SDS, BuOH and water reverse micelles at a fixed [H 2O]/[SDS] = 42.7 ratio, which allows variations in the percentage of BuOH in the system ranging 50–90%. For the purpose, pyrene was employed as fluorescent probe and N-cetylpiridinium chloride as static quencher. The results obtained show that N values, which range 8–33, show a linear variation with the amount of surfactant present in the system; meanwhile r wp values are mainly determined by the water content. The minimal water content and the minimal r wp value for reverse micelle formation were also estimated.

Positive cooperativity in substrate binding of human prostatic acid phosphatase entrapped in AOT–isooctane–water reverse micelles

The kinetics of 1-naphthyl phosphate and phenyl phosphate hydrolysis, catalyzed by human prostatic acid phosphatase (PAP) entrapped in AOT–isooctane–water reverse micelles, has been studied over surfactant hydration degree ( w 0) range 5 to 35. Continuous spectrophotometric acid phosphatase assays, previously prepared, were employed. PAP was catalytically active over the whole w 0 studied range. In order to determine steady-state reaction constants the experimental data were fitted to Hill rate equation. Positive cooperativity in substrate binding was observed, as it was earlier found in aqueous solutions. The extent of cooperativity (expressed as the value of the Hill cooperation coefficient h) increased from 1 to 4, when the micellar water-pool size was growing, at fixed enzyme concentration. In the plots of catalytic activity ( k cat) versus w 0, the maxima have been found at w 0=10 (pH 5.6) and 23 (pH 3.8). It is suggested that catalytically active monomeric and dimeric PAP forms are entrapped in reverse micelles of w 0=10 and 23, respectively.

Reorientation and vibrational energy relaxation of pseudohalide ions confined in reverse micelle water pools

Reorientation and vibrational energy relaxation times have been measured by ultrafast transient polarization IR spectroscopy for the antisymmetric stretching band in the 2000–2200 cm−1 region of pseudohalide ions, N3−, NCO−, and NCS−, confined in nanosize water pools of reverse micelles (RMs). The RMs are composed of nonionic nonylphenyl poly-oxyethylene surfactant in cyclohexane. Vibrational energy relaxation times (T1) of the NCO− and NCS− ions are about three times longer in small RMs compared to bulk water, similar to our earlier results on the N3− ion [J. Chem. Phys. 118, 7074 (2003)]. The longer T1 times are attributed to the reduced interaction between the ions and water molecules due to confinement effects and hydration of the surfactant headgroups by water molecules. Reorientation times (TR) of the N3− and NCS− ions are found to be more than one order of magnitude longer in small RMs than in bulk H2O. The observed longer TR times support the notion that water molecules inside RMs have very restricted mobility. As the water pool becomes larger with increasing water content, the interior water is less affected by the interface and bulk water properties reappear, leading to shorter T1 and TR times.

Dynamics of Polar Solvation in Quaternary Microemulsions

Steady-state spectroscopy and time-resolved solvation dynamics have been used to investigate the effects the addition of alkanol has on the location of coumarin 343 (C343) in cetyltrimethylammonium bromide (CTAB)/1-pentanol/cyclohexane/water, CTAB/1-heptanol/cyclohexane/water, and sodium dodecyl sulfate/1-heptanol/cyclohexane/water reverse micelles. Steady-state spectroscopy showed that the probe molecule is not sensitive to alkanol chain lengths or water loading. Instead, C343 is sensitive to the surfactant used to form the reverse micelles. Solvation dynamics measurements utilizing ultrafast time-resolved fluorescence-upconversion spectroscopy revealed two solvation reorganization times regardless of the micellar system studied or hydration level. A hundreds of picosecond time component is attributed to collective motion of the interface. Time-zero analysis reveals a substantial relaxation component occurring on the femtosecond time scale.

Synthesis of galactooligosaccharides and transgalactosylation modeling in reverse micelles

Galactooligosaccharides (GOS) were synthesized by β-galactosidase ( Escherichia coli) under controllable water concentration in reverse micelles. The products were precipitated by adding acetonitrile, and then separated by a semi-preparative HPLC column. Allolactose as one of the major products was purified and verified by the 13 C NMR analysis. The highest production of GOS was 29.7% (w/w) allolactose and 14% of other oligosaccharides under experimental conditions of pH 6.5, 37 °C and molar ratio of water to surfactant W o=19.32. Increasing W o decreased the production of GOS. Increasing temperature did not enhance the synthesis of GOS, but more allolactose (45%) could be produced at 45 °C, pH 6.5 and W o=19.32. This study proposes a transgalactosylation model that involves free water in reverse micelles and shows a good correlation with experimental data.

Synthesis and Stabilization of Silver Metallic Nanoparticles and Premetallic Intermediates in Perfluoropolyether/CO2 Reverse Micelle Systems

This article presents the stabilization of silver nanoparticle intermediates synthesized in ammonium perfluoropolyether (PFPE-NH4) reverse micelles with supercritical fluid (SCF) carbon dioxide solvent as the continuous phase. Specifically, the intermediates were formed by the reduction of silver nitrate salt (AgNO3) encapsulated within PFPE-NH4 reverse micelles. The effect of reducing agent type, reverse micelle water content, water core buffering, and bulk solvent type were all investigated as factors affecting stabilization of the silver nanoparticle intermediates. Particles were characterized by in situ UV−visible spectroscopy and transmission electron microscopy (TEM). The UV−vis spectrum of these nanosized silver particles is sensitive to particle size, and thus time-resolved spectral measurements were utilized as a means of monitoring both intermediate growth and persistence. The silver intermediates were stabilized in PFPE-NH4 reverse micelles as indicated by multiple UV−vis absorption bands that persist for periods of time measured to greater than 9 h. Intermediate stabilization is facilitated by a unique environment existing specifically as a result of PFPE-NH4 surfactant presence and its local water environment in the reverse micelle rather than any effects arising from the carbon dioxide solvent.

Molecular Dynamics Simulations of the Interior of Aqueous Reverse Micelles: A Comparison between Sodium and Potassium Counterions

We present the results of a molecular dynamics simulation study of the effects of counterion type on the properties of the interior region of aqueous reverse micelles. The model systems, which treat only the interior region at the atomistic level, are designed to represent water-in-oil microemulsions formed by the aerosol-OT surfactant, either with its usual counterion, Na+, or with the K+ counterion. Our study covers the water content, w0 = [H2O]/[surfactant], range of 1−7.5, where the reverse micelles are approximately spherical and contain tens to hundreds of water molecules in their interior pool. We find that several key structural and dynamical features of the reverse micelle water pool are strongly affected by counterion type. These effects can be ascribed to the differences in headgroup−counterion coordination and to the stronger affinity for water that the smaller Na+ ion exhibits. At low water content, K+ ions are able to coordinate four headgroups, while Na+ coordinates a maximum of three. As w0 increases, this coordination number decreases for both ions but always remains higher for K+, which also has a stronger tendency to remain in the interfacial region than does Na+. K+ displaces water from the interface to a larger extent than does Na+, with the result that fewer water molecules are trapped between the headgroups. That and the fact that K+ has a weaker attraction for water than does Na+ lead to higher mobility of water throughout the reverse micelle interior and more bulklike structural features, such as the number of water−water hydrogen bonds, in the interfacial region.

Vibrational Dynamics of the OH Stretching Mode of Water in Reverse Micelles Studied by Infrared Nonlinear Spectroscopy

ABSTRACTVibrational dynamics of the OH stretching mode of water in the water pool of reverse micelles (H2O/Aerosol OT (AOT)/isooctane) are studied by nonlinear infrared spectroscopy such as transient grating method and three-pulse photon echo peak shift measurements. The W0 value (W0 =[H2O]/[AOT]) is changed from 2 to 40, which corresponds to a water pool diameter of a few nm to about 20 nm. Polarization dependent transient grating experiments show rapid anisotropy decay of the OH stretching mode, which could be due to resonant intra and intermolecular energy transfer. From the three-pulse photon echo peak shift experiments, the spectral diffusion is found to be dependent on W0.

Intramolecular Charge Transfer and Solvation Dynamics of Coumarin 152 in Aerosol-OT, Water-Solubilizing Reverse Micelles, and Polar Organic Solvent Solubilizing Reverse Micelles

The relative retardation of intramolecular charge transfer (ICT) and the solvation dynamics of coumarin 152 in pure AOT, water, methanol, acetonitrile, and formamide reverse micelles have been investigated using picosecond time-resolved emission spectroscopy. The rate of ICT has been retarded almost 7 times at w0 = 4 and 4 times at w0 = 32 compared to that in pure water. The rate of retardation of ICT is also observed in the methanol and acetonitrile reverse micelles in comparison to that in pure methanol and acetonitrile. In pure AOT, the solvation time is 12.22 ns, but at w0 = 4 of water reverse micelles the solvation time is 7.27 ns. The slow dynamics in methanol, acetonitrile, and formamide reverse micelles is also observed. The relative retardation of the ICT rate is much smaller compared to the several thousand fold decrease in the solvation dynamics in the pool of the reverse micelles. The w dependency of solvation time is observed for water and methanol reverse micelles, but it is little for aceto...

Photophysics and photochemistry of hydrophilic cyanine dyes in normal and reverse micelles.

The photophysical and photochemical properties of the negatively charged hydrophilic cyanine dyes 1-3 were studied in isooctane/sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/water and cyclohexane-hexanol/Triton X-100 (TX100)/water reverse micelles and in aqueous solutions in the presence of surfactants. In spite of the opposite total charges, 2 and its structural analog, cationic 3,3'-diethylthiacarbocyanine (DTC), show similar photophysical and photochemical behaviour in AOT reverse micelles. However, in normal micelles their photophysical properties are quite different: 2 exhibits unusual behaviour [a sharp increase in the fluorescence quantum yield (phif) in the presence of both cationic and anionic micelles, but very small changes with neutral TX100], whereas the behaviour of DTC is normal. The phif values of 1-3 in AOT reverse micelles suffer a steep drop when w0 increases from 0 to 10 (w0 = [H2O]/[AOT]) and remain essentially constant at higher w0, while in TX100 reverse micelles the phif values undergo small variations with w0. A decrease in the photoisomerization quantum yield, and a rise in phif in S1 --> T intersystem crossing, in the back isomerization activation energy and in the preexponential factor were found for 1-3 in AOT reverse micelles with small w0. The unusual behaviour of 1-3 in micelles is attributed to the bipolar structure of the dyes.

Effects of normal and reverse micellar environment on the spectral properties, isomerization and aggregation of a hydrophilic cyanine dye

The spectral properties, cis→ trans isomerization and aggregation of the anionic hydrophilic cyanine 1, were studied in isooctane/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water, cyclohexane–hexanol/Triton X-100/water reverse micelles and in aqueous solutions in the presence of surfactants. The incorporation of 1 into micelles leads to dimer→monomer decomposition and cis→ trans isomerization. In AOT reverse micelles with w 0⩾5 ( w 0=[H 2O]/[AOT]) 1 forms J aggregates at concentrations much lower than one dye molecule per one micelle; at w 0=50 the most probable aggregation number is 3. Premicellar J aggregates of 1 are observed with both cationic and anionic surfactants, but not with Triton X-100.

The effect of anionic, cationic and neutral surfactants on the photophysics and isomerization of 3,3′-diethylthiacarbocyanine

The photophysical properties and isomerization of 3,3′-diethylthiacarbocyanine (DTC) were studied in isooctane/sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/water reverse micelles and in aqueous solutions containing sodium dodecylsulfate (SDS), cetyltrimethylammonium bromide (CTAB) and chloride (CTAC) and Triton X-100 (TX100) at concentrations below and above the critical micelle concentrations (c.m.c.). The absorption and fluorescence spectra of DTC in AOT reverse micelles are red-shifted and the nonradiative deactivation rate constant decreases strongly relative to that in water, leading to higher fluorescence quantum yields (Φf) and lifetimes. The DTC fluorescence decay in AOT reverse micelles and TX100 normal micelles is biexponential. SDS induces aggregation of DTC in water at concentrations well below the c.m.c. (10−5–10−3 mol l−1). Addition of SDS and TX100 at concentrations just above the c.m.c. increases Φf steeply, while CTAB (or CTAC) exerts such an effect only at concentrations much higher than the c.m.c. A rise in the isomerization activation energy and the pre-exponent is observed in the reverse micelles with small w0 (w0 = [water]/[AOT]); the isomerization rate constant decreases in SDS, TX100 and CTAC micelles.